Project Summary Rates of sexual violence victimization and perpetration among youth indicate a need for primary prevention (Kann et al., 2016; Ybarra & Mitchell, 2013). Sexual violence perpetration tends to co-occur with teen dating violence (TDV) and other types of interpersonal violence (Cutbush et al., 2016; Ozer et al., 2004; Sears et al., 2007), and males are more likely than females to be perpetrators (Ybarra & Mitchell, 2013). However, there are few evidence-based programs that address shared risk and protective factors to prevent sexual violence and other types of violence among young men. Existing programs for high school students are limited in that they consume several hours of class time and require broad student participation. There is a critical need to expand the list of evidence-based sexual violence primary prevention programs by rigorously evaluating programs that are feasible to implement and that fit with community priorities. The proposed research will rigorously evaluate the Men of Strength (MOST) Club, an after-school violence prevention curriculum that is grounded in a positive youth development framework and uses the Social Influence Model to engage young men as change agents in promoting healthy masculinity and peer leadership. This work will use innovative theoretical and methodological approaches to examine program effectiveness and mode and extent of information dissemination through peer leadership activities and peer networks. More specifically, the research agenda includes three aims that will (1) evaluate the effects of MOST Club on perpetration of sexual violence, TDV, other interpersonal violence, and bystander behavior, using a group RCT with 16 high schools; (2) examine theory-based mediators and moderators of program effects; and (3) evaluate peer leadership among MOST Club members by examining how, to what extent, and under what conditions Club members diffuse intervention messages to their peers. MOST Club has a well-developed curriculum and an existing implementation framework, highlighting the program's potential for scale-up and widespread dissemination if found to be effective. Findings from this study have the potential to move the field toward a positive youth development approach to achieving population-level reductions in sexual violence and co-occurring behaviors.